1. Field of the Invention
This invention relates to Fermi threshold Field Effect Transistors and more particularly to a Fermi threshold Field Effect Transistor (FET) with an abrupt channel doping profile and a low cut-off current flow.
2. Description of the Background
As will be appreciated by those skilled in the art, Fermi threshold, FETs are described in U.S. patents issued to Albert W. Vinal, including the following patents, which are incorporated herein by reference: U.S. Pat. Nos. 4,984,043; 4,990,974; 5,194,923; and 5,151,759. As in a conventional Metal On Oxide FET, the Fermi threshold FET has a source electrode, a drain electrode, a semiconductor channel between the source and drain where conduction can take place, and an insulated gate electrode which controls conduction in the semiconductor channel. The threshold voltage of an FET is the value of gate voltage needed to initiate device conduction. In a Fermi threshold FET of the type described in the above referenced patents, and to which this invention applies, the threshold voltage is set to twice the Fermi potential. The Fermi potential is that potential for which a semiconductor material has a probability of one half of being occupied by an electron. The criteria and equations for determining the parameters of a Fermi threshold FET, such as channel doping concentrations and depth, and source and drain doping, and gate structure for both N channel and P channel Fermi threshold FETs are described in the above referenced patents.
FIGS. 1 and 2 illustrate a conventional CMOS NFET with a uniform channel dopant. It will be understood that the same considerations apply to conventional PFETs and for more complex dopant profiles, such as retrograde well dopant structures. FIG. 1 shows a surface channel region of an NFET with its dopant concentration shown vertically and the depth from the wafer surface shown horizontally. The channel dopant is constant at 1E17 cm super3 of, in this example, Boron, which is consistent with the dopant level required for a 0.15 μm NFET device. When a positive bias is applied to the gate electrode, the surface of the silicone is depleted of its excess hole carriers, and the surface potential rises, as illustrated in FIG. 2. As the gate voltage is further increased, the surface depletion reaches a point where electrons begin accumulating at the surface of the wafer, forming an inversion layer, and conduction begins between the source and drain of the NFET. Because of the uniform dopant concentration in the wafer throughout the depletion region, the electron layer that forms is constrained by a large vertical electric field so that it is contained in a narrow region at the wafer surface (a few nm depth).
FIGS. 3 and 4 illustrates a prior art CMOS Fermi threshold FET. Source and drain regions are formed in the semiconductor substrate, typically a silicone bulk substrate, as illustrated in FIG. 3, or the substrate may be an epitaxial layer or a “tub” region formed in the bulk substrate. The source and drain regions are of the conductivity type opposite from the conductivity type of the substrate region itself, and have a higher donor concentration. Electrodes form external contacts for these regions. A thin gate oxide layer is formed on the surface of the substrate and a polysilicon gate contact is formed on the gate oxide, with a metal gate electrode. The substrate contact region is typically of the same conductivity type as the substrate but more highly doped. The Fermi channel has the same conductivity type as the source and drain and the opposite conductivity to the substrate. As will be appreciated by those skilled in the art, the depth and donor concentration of the Fermi channel are critical in forming a Fermi threshold FET.
Good performance of a Fermi threshold FET requires that a counter-doping layer (i.e. a layer doped opposite to the conventional channel doping) be inserted at the wafer surface. This counter-doping layer should have a very abrupt transition form the counter-doping layer impurity type to the conventional channel doping impurity type over a very short distance and the transition should be very close to the wafer surface. The counter-doping layer reduces the potential gradient, or reverse electric field, in the channel region in the silicon surface layer. FIG. 4 shows how the Fermi threshold FET has an altered electric field compared with that illustrated in FIG. 2. The Fermi threshold FET implements a narrow, abruptly terminated region of a dopant type opposite to the substrate (in the case illustrated here, N-type). This dopant region should be at a concentration level and depth such that all of the N-type dopant is depleted by the built-in potential of the N-P junction that forms because of N-type layer at the surface of the P-type substrate. This results in a surface region where any gate electrode voltage applied begins to build an inversion layer, and cause conduction between source and drain. An FET with such a very low threshold voltage (V subt) has a leakage current in its off (zero V subt) state. To lower the off state leakage current, the device V subt is increased by changing the work function of the gate electrode material. A gate electrode with a work function near the middle of the silicon band gap will increase the device Vsubt by 0.5 volts. Materials such as Tungsten (W) and Titanium Nitride (TiN) have such a work function. As illustrated in FIG. 4, to achieve a low cut off current, the entire potential distribution of the built-in junction in the silicon region should be contained in a shallow region near the wafer surface. This Fermi threshold device has a higher performance than the typical surface channel device because, as the channel turns on, the carriers are not constrained to flow up against the silicon surface as tightly, due to a lower vertical electric field at the surface compared with the uniform doped case illustrated in FIGS. 1 and 2. When the channel of the Fermi-FET is turned on, the charge carriers will flow through a deeper channel region and are not constrained to flow up against the silicon surface under the gate oxide where their mobility is degraded by surface scattering and high volume charge density effects. The effect of the counter-doping layer potential change on the device threshold voltage is counter balanced by a change in the gate electrode material work function. Moving the work function toward a mid-silicon band gap with a material such as W or TiN will restore the desired device threshold voltage.
The problem with extending the channel region depth into the wafer away from its surface is that when the gate electrode turns off the source drain current, the depth must not be so grate that the cannot adequately reduce the source drain current. Off state leakage current in a Fermi threshold FET is the source to drain current flowing in the FET when the gate voltage is zero. A criteria for a high performance FET is an off state leakage current of 1E−6 times the on state current, that is, the source drain current flowing with the Vdrain=Vgate=Vsupply. Gate electrode control when the gate electrode voltage is zero is key to limiting the leakage current. A problem in the Fermi threshold FET, with a counter doped channel, is the channel region extends into the wafer away from the wafer surface. Control of the lower boundary of the channel region in prior art Fermi threshold FETs has not been satisfactory in limiting the number charge carriers beyond the depth where the gate electrode can adequately reduce the source-drain current.
There have been prior art proposals to use Germanium as a charge neutral impurity in Silicon in order to reduce the diffusion motion of other impurity dopants in regions where the Ge resides, see for example U.S. Pat. Nos. 4,835,112, RE37,158, and 5,298,435. But these proposals do not address the problem of leakage current in the Fermi threshold FET with a counter doped channel region.
An object of this invention is the provision of an FET with a Fermi threshold voltage with high performance and low leakage current.
Another object of this invention is the provision of a method to form a shallow abrupt dopant transition region between the substrate, or well dopant type, and the surface counter doping layer of opposite type, with sufficient control to obtain the desired Fermi threshold voltage, low leakage current and high performance.